Conventionally, electric tools having a function of automatically locking an output shaft (spindle) when the motor is controlled to stop as described above are known (for example, see Patent Document 1 mentioned below).
The automatic lock mechanism of the electric tool described in Patent Document 1 is structured as follows. A projection formed on a circumference of an input shaft for inputting a rotation driving force and a projection formed on a circumference of an output shaft for outputting a rotation driving force are coupled with each other with a predetermined play angle. A pair of rollers are located between these projections within the play angle. One of the rollers is operable in correspondence with a rotation in a forward direction, and the other roller is operable in correspondence with a rotation in a reverse direction. On the side of the output shaft, a pair of wedge effect slopes are provided for providing a locking function with a wedge effect using one roller in the case of a rotation in the forward direction and the other roller in the case of a rotation in the reverse rotation. Thus, the lock mechanism is realized.
This electric tool operates as follows. The motor is controlled to stop, and the input shaft stops rotating. In this state, the operator pivots the output shaft by the play angle. Then, one of the pair of rollers bites into one of the wedge effect slopes corresponding to the rotation direction. Thus, the output shaft is locked by the wedge effect.
Such a lock mechanism using the rollers requires the rollers to rotate freely. Therefore, it is difficult to define the position at which each roller bites into the wedge effect slope. As a result, a problem may occur that the roller does not bite or bites insufficiently.
Instead of the rollers, a lock mechanism disclosed in Patent Document 2 is employable.
The lock mechanism described in Patent Document 2 operates as follows. A movable lock member (referred to as a “brake shoe” in Patent Document 2) movable in a radial direction is provided between an inner circumferential surface of a fixed ring which is fixed to a casing and an outer circumferential surface of a lock ring which is fixed to an output shaft. The movable lock member is pressed toward the fixed ring using a cam face formed on the outer circumferential surface of the lock ring. Thus, the output shaft is locked.
With a lock mechanism formed by such a movable lock member, when the relative rotation angle between the lock ring and the movable lock member is changed (when the relative rotation direction is changed), the movable lock member is pressed toward the fixed ring with certainty by the cam face. Therefore, the locking position is defined, which solves the above-mentioned problem of the lock mechanism using the rollers.
Patent document 1: Japanese Publication for Opposition No. 6-53350
Patent document 2: Japanese Laid-Open Patent Publication No. 2002-337062